A deep search for the host galaxies of gamma-ray bursts with no detected optical afterglow^⋆,⋆⋆

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
e-mail: rossi@tls-tautenburg.de
² Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain
⁴ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁵ University of Rochester, Department of Physics and Astronomy, Rochester, NY 14627-0171, USA
⁶ Florida Institute of Technology, Melbourne, FL 32901, USA
⁷ Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA
⁸ Department of Physics and Astronomy, Dickinson College, Carlisle, PA 17013, USA
⁹ Universe Cluster, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
¹⁰ Dark Cosmology Centre, Niels Bohr Institute, Univ. of Copenhagen, Juliane Maries Vej 30, 2100 Købnhaven, Denmark
¹¹ INAF-IASF Bologna, Area della Ricerca CNR, via Gobetti 101, 40129 Bologna, Italy
¹² Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
¹³ American River College, Physics Dpt., 4700 College Oak Drive, Sacramento, CA 95841, USA
¹⁴ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18.008 Granada, Spain
¹⁵ Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horská 3a/22, 12800 Prague, Czech Republic
¹⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo Fermi 5, 50125 Firenze, Italy
¹⁷ Institute of Astronomy, University of Cambridge, Madingley Road CB3 0HA, Cambridge, UK
¹⁸ Universitá degli Studi di Milano-Bicocca, Piazza della Scienza 2, 20126 Milano, Italy
¹⁹ Scuola Normale Superiore di Pisa – Piazza dei Cavalieri 7, 56126 Pisa, Italy
²⁰ INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy

Received: 6 May 2011
Accepted: 5 June 2012

Abstract

Context. Gamma-ray bursts (GRBs) can provide information about star formation at high redshifts. Even in the absence of a bright optical/near-infrared/radio afterglow, the high detection rate of X-ray afterglows by Swift/XRT and its localization precision of 2–3 arcsec facilitates the identification and the study of GRB host galaxies.

Aims. We focus on the search for the host galaxies of 17 bursts with arcsec-sized XRT error circles but no detected long-wavelength afterglow, in spite of their deep and rapid follow-up observations. Three of these events can also be classified as truly dark bursts, i.e., the observed upper limit on the optical flux of the afterglow was less than expected based on the measured X-ray flux. Our goals are to identify the GRB host galaxy candidates and characterize their phenomenological parameters.

Methods. Our study is based on deep R_C and K_s-band observations performed with FORS1, FORS2, VIMOS, ISAAC, and HAWK-I at the ESO/VLT, partly supported by observations with the seven-channel imager GROND at the 2.2-m telescope on La Silla, and supplemented by observations with NEWFIRM at the 4-m telescope on Kitt Peak. To be conservative, we searched for host galaxy candidates within an area of twice the radius of each associated 90% c.l. Swift/XRT error circle.

Results. For 15 of the 17 bursts, we find at least one galaxy within the searching area, and in the remaining two cases only a deep upper limit to R_C and K_s can be provided. In seven cases, we discover extremely red objects in the error circles, at least four of which might be dust-enshrouded galaxies. The most remarkable case is the host of GRB 080207, which has a color of (R_C − K_s)_AB ~ 4.7 mag, and is one of the reddest galaxies ever associated with a GRB. As a by-product of our study we identify the optical afterglow of GRB 070517.

Conclusions. Only a minority of optically dim afterglows are due to Lyman dropout (≲ 1/3). Extinction by dust in the host galaxies might explain all other events. Thereby, a seemingly non-negligible fraction of these hosts are globally dust-enshrouded, extremely red galaxies. This suggests that at least a fraction of GRB afterglows trace a subpopulation of massive starburst galaxies, which are markedly different from the main body of the GRB host galaxy population, namely the blue, subluminous, compact galaxies.